1. Field Of The Invention
The present invention relates to apparatus and associated method for the chromatographic separation of materials and, more specifically, it focuses primarily upon supercritical fluid chromatographic separation.
2. Description Of The Prior Art
It has long been known to employ chromatography as a means for separating constituents from a sample and evaluating the same. Generally, a sample is introduced into a packed or open tubular column in a mobile phase which is adapted to interact with the materials contained in the column in order to effect a separation which may be analyzed by a detector positioned at the outlet end of the column.
It has been known to employ gas chromatography, liquid chromatography and supercritical fluid chromatography which is sometimes known as dense gas chromatography. See, generally, Grob, Modern Practice of Gas Chromatography, 2nd Edition, John Wiley & Sons (1985); Jorgenson et al., New Techniques for Liquid Chromatography in Open-Tubular Columns, Journal of Pharmaceutical & Biomedical Analysis, Vol. 2, No. 2, pages 191-196, (1984) and U.S. Pat. No. 4,479,380.
It has also been known to employ capillary columns in various types of chromatography including packed and unpacked column supercritical fluid chromatography, Hartmann, Fluid Chromatography of Styrene Oligomeres (1977) and U.S. Pat. No. 4,479,380. See also, U.S. Pat. No. 4,124,358.
It has been known in connection with chromatography to employ flame detectors. See generally, U.S. Pat. No. 3,790,348. Many types of detectors employed with supercritical fluid chromatography such as flame ionization detectors, nitrogen-phosphorous detectors, and flame photometric detectors operate at or near atmospheric conditions, while other detectors such as mass spectrometric operate at less than atmospheric conditions. As a result, it becomes desirable in supercritical fluid chromatography to reduce the pressure of the column effluent to atmospheric pressure. Highly sensitive detection of substances separated in the chromatograph have been detected by flame detectors. This is accomplished by ionizing the column effluent in a detector chamber. The chamber consists of an air/hydrogen flame and two electrodes, one above and one below the flame. The electrode above the flame collects the resulting electrons and amplifies the ionization current generated for delivery to the input of an electrometer amplifier.
Various aspects of the column-detector interface have been recognized. It has been known that problems are associated with depressurization and solvent cluster formation. Giddings et al., Dense-Gas Chromatography at Pressures up to 2000 Atmospheres, Jour. of Chromatographic Science (1969); Jentoft et al., Pressure-Programmed Supercritical Fluid Chromatography of Wide Molecular Weight Range Mixtures, Jour. of Chromatographic Science, p. 138 (1970). It has been known to attempt to employ pinched platinum irridium tubing, Smith et al., Performance of Capillary Restrictors in Supercritical Fluid Chromatography, Analytical Chemistry (1986), pin holes in disks and straight walled capillary restrictors, U.S. Pat. No. 4,479,380. It has also been known to use tapered restrictors, large interface heating zones and increasing the pressure drop by coating the capillary tubing with a polymer, Richter et al., Modified Flame Ionization Detector for Analysis of Large Molecular Weight Polar Compounds by Capillary Supercritical Fluid Chromatography, Jour. of High Resolution Chromatography & Chromatography Communications (1985). These prior art disclosures were directed toward achieving smoother depressurization and the ability to heat the depressurized stream better. This was accomplished by employing thin walled capillary tubing or using tapered capillary tubing and/or using additional heating zones to facilitate better heat transfer properties to carbon dioxide. Various forms of fittings have been known in chromatography. See, for example, U.S. Pat. No. 4,083,702.
In spite of these known flame detector disclosures, here remains a significant problem in that the solution of the substances in the carrier gas tends to become unstable through the expansion and substances conglomerate to flocculate or other aggregates which can become so large that a visible veil may appear. With higher substance concentration, condensation may occur. The undesired result of such action is that the peaks are distorted with jagged spikes. These distortions are attributable to the bursting of the previously flocculated substance particles in the flame. This renders the reproduction of the measurement difficult if not impossible.
It has been suggested to operate the flame ionization detector at the pressure of the separating column, but this is not practical as the control of the fuel gases and of the flame becomes much too complicated. Decreasing of column pressure prior to introduction into the flame ionization detector has tended to result in undesirable flocculation. In order to achieve desired smooth depressurization, solutes that do precipitate are swept through into the detector. The mobile phase should not be allowed to condense before detection as this would result in ion bursts that would produce extra electronic signals in the detector. Another problem with high pressure fluid depressurization is that it reduces the temperature of the fluid and flame. In some cases such as with carbon dioxide, the expansion of the fluid may actually lead to a two phase region. Formation of liquid or solid phases could lead to sudden expansions in the flame leading to spikes in the signal and instability of the flocculant resulting in spike output. Further, variation in the flame temperature introduces noise in the baseline of the output.
It has been known to modify the fluid flow rate by controlling the pressure drop through introduction of a backpressure into the fluid flow upstream of the nozzle. See P. A. Peaden, et al., Anal. Chem. 54, 1090-1093 (1982). The column in the oven is connected to a restrictor and the pressure downstream of the restrictor and upstream of the nozzle is maintained in any desired pressure which is typically slightly above the critical pressure through the introduction of very high pressure nitrogen. The maximum pressure nitrogen was limited to 1500 psia. This maximum is quite low and would limit the range of flow and therefore was not widely used.
It has not been known to control simultaneously pressure and flow programming capability with a wide range of flow in supercritical fluid chromatography. As a result, certain analysis are not attainable with the known means.
Flow control in gas chromatography through progressive increasing in pressures and mixing of effluent gas has been suggested. See U.S. Pat. No. 3,250,057.
It has been known to employ pressure programming in supercritical chromatography. See, Novotny et al., Temperature and Pressure effects in Supercritical Fluid Chromatography, Jour. of Chromatographic Science, p. 17 (1971). See also, U.S. Pat. No. 3,646,950 which, while not relating to chromatography, suggests pressure alterations to control fluid flow rates.
It has been known in supercritical fluid chromatography to maintain the flow rate constant during pressure programming by employing two pumps, one of which delivers makeup flow to maintain backpressure. Associated restrictors are employed. See Hirata et al., Control of Flow Rate in Supercritical Fluid Chromatography, p. 627, Chromatography, Vol. 21, No. 11 (1986). This approach requires the use of two pumps, manual coordination therebetween and does not employ information feedback in effecting control. See also U.S. Pat. No. 4,479,380 which discloses the use of two gas tanks in open-tubular supercritical chromatography.
A fluid with its temperature and pressure near the critical points is known as a supercritical fluid. Under such conditions, the density of the fluid is similar to that of a liquid, but the mass transfer properties are in between those of liquids and gases. The diffusion coefficient of the solute in the supercritical fluid is on the order of twice the magnitude of those in liquids.
In chromatography it is often necessary to control the flow of the fluid as well as the pressure. This is particularly true in supercritical fluid chromatography.
Controlling the density of a supercritical fluid by pressure programming has been known. See R. E. Jentoft et al., Journal in J. Chromatogr. Sci. 8, 1970, pages 138-142; J. E. Conaway et al., J. Chromatogr. Sci. 16, 1978, pages 102-110; and U.S. Pat. No. 4,479,380.
Temperature programming has been used in gas chromatography to change the solute retention time in the column. Flow programming has been used in liquid chromatography and has some relationship conceptually to the temperature programming in gas chromatography. It has also been known to use flow programming in packed supercritical fluid chromatography, but it has not been believed to have been used in capillary supercritical fluid chromatography.
In spite of the systems known to the prior art, there remains a very real and substantial need for an improved system for use in supercritical fluid chromatography to a more efficiently separate solute with a high degree of resolution.